Abstract

Great Sand Dunes National Monument is located in south-central
Colorado along the eastern edge of the San Luis Valley. The Great Sand
Dunes National Monument contains the tallest sand dunes in North America;
some rise up to750 feet. Important ecological features of the Great Sand
Dunes National Monument are palustrine wetlands associated with interdunal
ponds and depressions along the western edge of the dune field. The existence
and natural maintenance of the dune field and the interdunal ponds are
dependent on maintaining ground-water levels at historic elevations. To
address these concerns, the U.S. Geological Survey conducted a study,
in collaboration with the National Park Service, of ground-water flow
direction, water quality, recharge sources, and age at the Great Sand
Dunes National Monument.

A shallow unconfined aquifer and a deeper confined aquifer
are the two principal aquifers at the Great Sand Dunes National Monument.
Ground water in the unconfined aquifer is recharged from Medano and Sand
Creeks near the Sangre de Cristo Mountain front, flows underneath the
main dune field, and discharges to Big and Little Spring Creeks. The percentage
of calcium in ground water in the unconfined aquifer decreases and the
percentage of sodium increases because of ionic exchange with clay minerals
as the ground water flows underneath the dune field. It takes more than
60 years for the ground water to flow from Medano and Sand Creeks to Big
and Little Spring Creeks. During this time, ground water in the upper
part of the unconfined aquifer is recharged by numerous precipitation
events. Evaporation of precipitation during recharge prior to reaching
the water table causes enrichment in deuterium (2H) and oxygen-18 (18O)
relative to waters that are not evaporated. This recharge from precipitation
events causes the apparent ages determined using chlorofluorocarbons and
tritium to become younger, because relatively young precipitation water
is mixing with older waters derived from Medano and Sand Creeks.

Major ion chemistry of water from sites completed in the
confined aquifer is different than water from sites completed in the unconfined
aquifer, but insufficient data exist to quantify if the two aquifers are
hydrologically disconnected. Radiocarbon dating of ground water in the
confined aquifer indicates it is about 30,000 years old (plus or minus
3,000 years). The peak of the last major ice advance (Wisconsin) during
the ice age occurred about 20,000 years before present; ground water from
the confined aquifer is much older than that. Water quality and water
levels of the interdunal ponds are not affected by waters from the confined
aquifer. Instead, the interdunal ponds are affected directly by fluctuations
in the water table of the unconfined aquifer. Any lowering of the water
table of the unconfined aquifer would result in an immediate decrease
in water levels of the interdunal ponds. The water quality of the interdunal
ponds probably results from several factors, including the water quality
of the unconfined aquifer, evaporation of the pond water, and biologic
activity within the ponds.